Such parts are often made of steel that is quenched and annealed, or where possible, out of forged steel of ferrito-perlitic structure which is believed to offer the best technical and economic compromise, even though its mechanical performance is nevertheless limited.
Ferrito-perlitic structure steels that are often used for this purpose are of types XC70, 45Mn5, 30MnSiV6, and 38MnSiV5, and after rolling or forging they are simply subjected to in-line cooling in still air. They are thus relatively economic to produce, however their lifetime in the presence of high levels of stress is limited.
It has already been proposed to make such parts out of bainite steel using a grade of the 25MnSiCrVBS type, with cooling after forging or rolling taking place in air. Strength performance compared with the above examples is considerably improved, but nevertheless remains relatively limited compared with that which can be achieved using a steel that has been quenched and annealed.